An air-fuel ratio control device of an internal combustion engine comprising combustion chambers and fuel injectors each arranged corresponding to each combustion chamber is described in the unexamined Japanese Patent Publication No. 2008-38785. In this air-fuel ratio control device, an air-fuel ratio of a mixture gas formed in each combustion chamber is estimated, the average value of the estimated air-fuel ratios is calculated as an average air-fuel ratio, this calculated average air-fuel ratio is set as a target air-fuel ratio, and the air-fuel ratio of the mixture gas formed in each combustion chamber is controlled to the target air-fuel ratio by amending an amount of a fuel supplied from each fuel injector to the corresponding combustion chamber such that the air-fuel ratio of the mixture gas formed in each combustion chamber becomes the target air-fuel ratio.